1. Field of the Invention
The invention relates to a hydraulic down-hole motor of peristaltic type for use in underground drilling operations.
2. Description of the Prior Art
Down-hole motors are used in drilling operations for driving a drill bit or other rotary tools. The motors usually comprise a stator which is suitable to be coupled to the lower end of a drill string and a rotor which is driven by the flow of drilling liquid to the drill bit. Examples of such motors are the hydraulic turbines and the positive-displacement motors, such as the vane motor. An advantage of the positive-displacement motors over the turbines is that the speed of rotation of the rotor can be controlled accurately by varying the flow of drilling fluid through the drill string. A drawback of the currently used down-hole motors of the positive-displacement type is that in particular those parts of the stator and rotor that contact each other are subject to high wear, such that the operational period of these motors is rather low.
A positive-displacement motor of the peristaltic type has the advantage over other types of positive-displacement motors in that wear of the motor during operation thereof is extraordinarily low. Motors of the peristaltic type are known per se. A motor of this type comprises a series of "breathing" working chambers that are formed between an impermeable membrane and a tubular section of the stator housing. The rotor comprises a set of rollers that press the membrane at selected intervals against the tubular stator section. The working chambers are designed such that during operation of the motor the rollers are driven to ride upon the membrane in front of the expanding portions of the chambers, thereby rotating the rotor relative to the stator. Such peristaltic motors are disclosed in the specifications of U.S. Pat. No. 3,039,442 entitled "Motor Operative By Action Of A Fluid Expansible Membrane", filed Dec. 1, 1960 by Hanns Hornschuch and Jack R. Webb, Ser. No. 73,039, and of USSR Pat. No. 595535. The known peristaltic motors, however, are designed for above ground operations and are not suitable to be used down-hole due to their general configuration, in particular due to their low volume-diameter ratio.
It is to be understood that down-hole motors should have typically a rather high volume-diameter ratio since considerable power is required to drive a drill bit which requires a motor with a large displacement volume, whereas the motor is to be inserted in small-diameter boreholes.
It is virtually impossible to increase the volume-diameter ratio of the known peristaltic motors to a sufficient extent for downhole use thereof, since in these motors the rollers are mounted on satellite shafts that are supported at the ends thereof by bearings being carried by a central rotor body. A large increase of the volume-diameter ratio of the motors would require the use of elongate rollers and satellite shafts. Such elongate shafts might easily be bent under the forces exerted thereon by the rollers, which would impair the proper operation of the motor.